Magnetic field sensors including a magnetic field sensing element, or transducer, such as a Hall Effect element or a magnetoresistive element, are used in a variety of applications to detect aspects of movement of a ferromagnetic article, or target, such as proximity, speed, and direction. Illustrative applications include, but are not limited to, a magnetic switch or “proximity detector” that senses the proximity of a ferromagnetic article, a proximity detector that senses passing ferromagnetic articles (for example, magnetic domains of a ring magnet or gear teeth), a magnetic field sensor that senses a magnetic field density of a magnetic field, and a current sensor that senses a magnetic field generated by a current flowing in a current conductor. Magnetic field sensors are widely used in automobile control systems, for example, to detect ignition timing from a position of an engine crankshaft and/or camshaft, and to detect a position and/or rotation of an automobile wheel for anti-lock braking systems.
In applications in which the ferromagnetic target is magnetic, a magnetically permeable concentrator or magnetic flux guide is sometimes used to focus the magnetic field generated by the target on the magnetic field transducer, thus increasing the sensitivity of the sensor, allowing the use of a smaller magnetic target, and/or allowing the magnetic target to be sensed from a larger distance (i.e., a larger airgap). In other applications, in which the ferromagnetic target is not magnetic, a permanent magnet, sometimes referred to as a back bias magnet, may be used to generate the magnetic field that is then altered by movement of the target.
In some applications it is desirable to provide a back bias magnet with two magnetic poles on the magnet surface adjacent to the magnetic field transducer. For example, as described in a U.S. Pat. No. 5,781,005 entitled “Hall-Effect Ferromagnetic-Article-Proximity Sensor,” which is assigned to the Assignee of the subject application, the near presence of opposite poles serves to short out the lines of flux when no ferromagnetic article is present, thereby presenting a significant and easily recognizable difference between an article present (e.g., gear tooth present) condition and an article absent (e.g., gear valley present) condition and maintaining a low magnetic flux density baseline regardless of airgap. Because of the easily recognizable difference in the magnetic field signal, these types of arrangements are advantageous for use in sensors in which it is necessary to detect the presence/absence of a magnetic article, such sensors sometimes being referred to as True Power On Sensors, or TPOS, sensors.
Generally, back bias magnets and concentrators are held in place relative to the magnetic field sensing element by mechanical means, such as an adhesive as shown in a U.S. Pat. No. 6,265,865 entitled “Single Unitary Plastic Package for a Magnetic Field Sensing Device,” which is assigned to the Assignee of the subject application. Such mechanical positioning can lead to performance variations, such as sensitivity variations, from device to device due to position tolerances. Thus, it is advantageous to manufacture the sensor so that the sensor and the back bias magnet or concentrator are integrally formed, thereby eliminating position tolerances. A magnetic field sensor of this type is described in a U.S. Patent Application Publication No. 2010/0141249 entitled “Magnetic Field Sensors and Methods for Fabricating the Magnetic Field Sensors,” which is also assigned to the Assignee of the subject application and in which a concentrator or magnet may be formed by a liquid encapsulant or a combination of a liquid encapsulant and permanent magnet in a cavity on the side of the sensor opposite the target.
While the use of a back bias magnet is advantageous in certain applications, the hard magnetic material used to form the magnet is relatively expensive and represents a significant part of the overall cost of the sensor.
There are many package types and fabrication techniques in use for providing integrated circuit magnetic field sensors. For example, the semiconductor die in which the magnetic field sensing element is formed may be attached to a lead frame by various techniques, such as with an adhesive tape or epoxy, and may be electrically coupled to the lead frame by various techniques, such as with solder bumps or wire bonding. Also, the lead frame may take various forms and the semiconductor die may be attached to the lead frame in an orientation with the active semiconductor surface (i.e., the surface in which the magnetic field sensing element is formed) being adjacent to the lead frame in a so called “flip-chip” arrangement, with the active semiconductor surface opposite the lead frame surface in a so called “die up” arrangement, or with the semiconductor die positioned below the lead frame in a so called “Lead on Chip” arrangement.
Molding is often used in fabricating integrated circuit magnetic field sensors to provide the protective and electrically insulative “overmold” to the semiconductor die. Transfer molding has also been used to form two different molded portions for various reasons. For example, in a U.S. Pat. No. 7,816,772 entitled “Methods and Apparatus for Multi-Stage Molding of Integrated Circuit Package” which is assigned to the Assignee of the subject application, a first molded structure is formed over the semiconductor die to protect wire bonds and the device is overmolded with a second molded structure formed over the first molded structure. In a U.S. Patent Application Publication No. 2009/0140725 entitled “Integrated Circuit Including Sensor having Injection Molded Magnetic Material,” an injection molded magnetic material encloses at least a portion of a magnetic field sensor.
Molding, while providing a cost effective fabrication technique, can present challenges, such as removal of the device from the mold in a manner that does not subject the device to deleterious stresses.